Black Box in green screen out of NOWHERE after it's already working? Please help!

Hello, I am using Final Cut Pro X. I am keying and it goes great. Then while I'm editing, a black box covers part of my screen when I haven't done anything to cause that in my mind. Just normal editing. Then I click "undo", even if it's just a simple cut, and it fixes it. It even happens if I don't touch anything and just allow the computer to sit. If I simply adjust any parameter, it fixes itself, but then seems to go back in just a few minutes or seconds. Everything is up to date. I have watched numerous videos on green screen and follow them to a tee. I also have not found anyone with a similar issue! It very much seems like a bug. I'm on the m1 iMac first gen. It's just happening out of nowhere! Please help!